OCR A Level Biology

Revision Notes

6.2.6 Predicting Inheritance: Identifying Linkage

Predicting Inheritance: Identifying Linkage

Autosomal linkage

  • Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis
  • However, not all genes assort independently during meiosis
  • Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination
  • Linkage between genes affects how parental alleles are passed onto offspring through the gametes
  • When writing linked genotypes it can be easier to keep the linked alleles within a bracket
    • For example, an individual has the genotype FFGG. However, if there is linkage between the two genes, it would be written as (FG)(FG)

Identifying autosomal linkage from phenotypic ratios

  • In the following theoretical example, a dihybrid cross is used to show the inheritance of two different characteristics in a species of newt
    • In this theoretical example, the two genes responsible for the characteristics (each gene consisting of a pair of alleles) are inherited independently of one another because the two genes are located on different non-homologous chromosomes (i.e. the genes are unlinked)
  • The genes are for tail length and scale colour
  • The gene for tail length has two alleles:
    • Dominant allele T produces a normal length tail
    • Recessive allele t produces a shorter length tail
  • The gene for scale colour has two alleles:
    • Dominant allele G produces green scales
    • Recessive allele g produces white scales
  • A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes:    TtGg                                    ttgg

Parental gametes:       TG or Tg or tG or tg             tg

  • The outcomes for this dihybrid cross if the genes are unlinked are as follows:
    • Predicted ratio of phenotypes in offspring = 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales
    • Predicted ratio of genotypes in offspring = 1 TtGg : 1 Ttgg : 1 ttGg : ttgg
  • However, if the same dihybrid cross is carried out but this time the genes are linked, we get a different phenotypic ratio
    • If the genes are linked, we get a 1 : 1 phenotypic ratio (the phenotypes of the offspring will be 1 normal tail, green scales : 1 short tail, white scales)
    • This change in the phenotypic ratio occurs because the genes are on the same chromosome
    • The unexpected phenotypic ratio therefore shows us that the genes are linked
  • The explanation for this new phenotypic ratio is given in the worked example below:

Worked Example

Worked example: Explaining autosomal linkage

  • In reality, the genes for tail length and scale colour in this particular species of newt show autosomal linkage
  • Again, as in the theoretical example given above (in which the genes were not linked), a newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes:    (TG)(tg)                 (tg)(tg)

Parental gametes:       (TG) or (tg)            (tg)

Dihybrid Cross with Linkage Punnett Square Table

Dihybrid Cross with Linkage Punnett Square table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring = 1 normal tail, green scales : 1 short tail, white scales
  • Predicted ratio of genotypes in offspring = 1 (TG)(tg) : 1 (tg)(tg)

Sex-linkage

  • Sex-linked genes are only present on one sex chromosome and not the other
  • This means the sex of an individual affects what alleles they pass on to their offspring through their gametes
  • If the gene is on the X chromosome males (XY) will only have one copy of the gene, whereas females (XX) will have two
  • Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
  • Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
  • A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
  • If that offspring is a male, he will have the disease
  • For sex-linked genes that occur on the X chromosome, there are three phenotypes for females: ‘normal’, ‘carrier’ and ‘has the disease’, whereas males have only two phenotypes: ‘normal’ or ‘has the disease’
  • The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:

X-linked genetic cross, IGCSE & GCSE Biology revision notes

Punnett square showing the inheritance of colourblindness, an X-linked condition

  • In the cross above, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male

Worked Example

Worked example: Sex-linkage

  • Haemophilia is a well known sex-linked disease
  • There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
  • There are two alleles for factor VIII, the dominant F allele which codes for normal factor VIII and the recessive f allele which results in a lack of factor VIII
  • When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can’t clot normally
  • The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia

Parental phenotypes: carrier female x normal male

Parental genotypes:      XFXf                              XFY

Parental gametes:      XF or Xf                        XF or Y

Monohybrid Cross with Sex-linkage Punnett Square Table

Monohybrid Crosses_1, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring = 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting
  • Predicted ratio of genotypes in offspring = 1 XFXF : 1 XFXf : 1 XFY : 1 XfY

Epistasis

  • In some cases, one gene can affect the expression of another gene
  • Epistasis is when two genes on different chromosomes affect the same feature
  • If epistasis is present it needs to be taken into account when determining the phenotypes of individuals
  • The whole combination of alleles from the different genes dictates the phenotype

Worked Example

Worked example: Explaining epistasis

  • There is a gene that dictates the feather colour of pigeons
  • The gene has two alleles (R or r):
    • Allele R codes for a pigment that produces grey feathers
    • Allele r doesn’t produce a pigment, resulting in white feathers
  • Another gene has also been found to have an effect on feather colour
  • This gene has two alleles (F or f):
    • Allele F codes for the production of an enzyme that stops grey feathers from being produced even if the allele R is present
    • Allele f doesn’t produce an enzyme
  • The possible phenotypes are, therefore, as follows:
    • RRFF (white feathers)
    • RrFF (white feathers)
    • rrFF (white feathers)
    • RRFf (white feathers)
    • RrFf (white feathers)
    • rrFf (white feathers)
    • rrff (white feathers)
    • RRff (grey feathers)
    • Rrff (grey feathers)

Identifying epistasis from phenotypic ratios

  • In the same way that a deviation from the expected phenotypic ratios suggests that there is linkage (i.e. the genes being inherited are linked), phenotypic ratios can also be used to identify if epistasis may be occurring
  • The example that is given above (of feather colour in pigeons) can be used to demonstrate this by comparing the phenotypic ratios of offspring when epistasis is not occurring with the phenotypic ratios of offspring when epistasis is occurring

Without epistasis

  • If there is no epistasis, the second gene does not affect the expression of the first gene
  • If two heterozygous pigeons (RrFf) are crossed with each other and no epistasis is occurring (i.e. if the enzyme coded for by F does not affect feather colour), we would expect the following:

Parental phenotypes: grey feathers, produces enzyme x grey feathers, produces enzyme

Parental genotypes:   RrFf                                     RrFf

Parental gametes:         RF or Rf or rF or rf                RF or Rf or rF or rf

  • As allele F does not affect feather colour, the genotypes and phenotypes of offspring are as follows:
    • RRFF (grey feathers)
    • RrFF (grey feathers)
    • rrFF (white feathers)
    • RRFf (grey feathers)
    • RrFf (grey feathers)
    • rrFf (white feathers)
    • rrff (white feathers)
    • RRff (grey feathers)
    • Rrff (grey feathers)
  • Predicted ratio of phenotypes in offspring = 6 grey feathers : 3 white feathers

With epistasis

  • If epistasis is occurring, the second gene does affect the expression of the first gene
  • If the same dihybrid cross is carried out (two heterozygous pigeons (RrFf) are crossed with each other) and epistasis is occurring (i.e. if the enzyme coded for by F affects feather colour), we get a different phenotypic ratio
  • In this case, we would expect the following genotypes and phenotypes of offspring:
    • RRFF (white feathers)
    • RrFF (white feathers)
    • rrFF (white feathers)
    • RRFf (white feathers)
    • RrFf (white feathers)
    • rrFf (white feathers)
    • rrff (white feathers)
    • RRff (grey feathers)
    • Rrff (grey feathers)
  • Predicted ratio of phenotypes in offspring = 2 grey feathers : 7 white feathers
    • In this example, the occurrence of epistasis has greatly changed the phenotypic ratios of the offspring
    • Unexpected phenotypic ratios can therefore be used to help identify whether or not epistasis may be occurring

Exam Tip

When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents).

However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced.  This is due to the crossing over that occurs during meiosis. The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents.

So if a question comes along that asks you why recombinant offspring are present you now know why!

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